The present invention relates generally to semiconductor devices, and more specifically, to a structure and method for forming electrical connections between levels of a semiconductor device.
It is a common practice in the fabrication of integrated circuits to make use of an interconnection layer of aluminum, which is highly conductive while being relatively easy to deposit and etch, for connecting different elements of the integrated circuit to each other. This layer usually rests on an insulating layer which in turn is located above a conductive layer. The insulating layer is opened prior to deposition of the aluminum in order to expose the conductive layer surfaces with which to establish a metallic interconnection. These conductive surfaces can be monocrystalline silicon surfaces (transistor sources, drains, collectors, bases and emitters), polycrystalline silicon elements (field-effect transistor gates) or metallic surfaces of another interconnection layer. The contact opening is initially filled or xe2x80x9cpluggedxe2x80x9d with a metallic layer, such as aluminum or tungsten, to make a solid electrical connection between the underlying conductive layer and the overlying interconnection aluminum layer.
However, problems occur with aluminum contact to silicon because of interdiffusion of aluminum in later process steps and may cause spiking in the silicon. To prevent spiking and alloying between an aluminum interconnection metal and silicon surface, a thin xe2x80x9cbarrierxe2x80x9d or nucleation layer is deposited on the exposed silicon surface of the contact opening prior to filling the opening with aluminum. The most useful and practical barrier metal has been a titanium, or titanium nitride on titanium (TiN/Ti) bilayer which deposits well on silicon surfaces and also acts as glue for metal plugs of tungsten on silicon surfaces. The TiN/Ti scheme, however, has one major drawback. It does not deposit well on sidewalls of an opening. The bottom and sidewall coverage is especially important in xe2x80x9chot aluminum plugxe2x80x9d processing in order to facilitate the surface diffusion of aluminum atoms into the contact opening and to withstand an even greater tendency for spiking due to the high aluminum deposition temperature. In order to ensure adequate deposition of the barrier film on the bottom and sidewalls of a contact opening for such aluminum plugs, it was previously necessary to step down the contact opening as shown in U.S. Pat. No. 4,592,802. However, such steps of the contact opening uses valuable layout space of a silicon structure.
This problem is partly solved by the use of chemical vapor deposition (CVD) tungsten etched back plug, a process well known in the art and described in U.S. Pat. No. 4,592,802, incorporated herein by reference. In this process, the contact opening is filled with tungsten to a level above the insulating layer to make sure the opening is completely filled. The deposited tungsten is then etched back without a mask until the insulating layer is exposed. Because CVD of tungsten is conformal, i.e., the deposition rate on vertical surfaces is the same as that of horizontal surfaces, this method produces a complete plug in the opening. By using this method, a barrier metal layer is no longer needed to prevent the problem of spiking between an aluminum plug and silicon.
Despite the success achieved with tungsten plugs in preventing spiking, however, it is still desirable to have a barrier metal layer in the contact opening. In order to uniformly deposit tungsten on an insulation layer with contact holes therein, it is strongly preferred to have a nucleation layer, usually of a barrier metal material or other glue material to ensure uniform deposit on all surfaces. Moreover, the barrier metal layer is desirable to prevent a phenomenon known as xe2x80x9ctungsten encroachmentxe2x80x9d or xe2x80x9cworm holing.xe2x80x9d Tungsten and silicon do not readily react at typical metallization temperatures of less than 500xc2x0 C. However, the CVD of tungsten is performed using WF6 and the fluorine in WF6 reacts with silicon in the presence of tungsten which acts as a catalyst in the reaction. This tungsten encroachment problem is well known in the art and widely reported in industry literatures. A barrier metal layer such as TiN solves this problem by preventing fluorine from contacting silicon surfaces.
While the barrier metal is needed only at the bottom of the contact opening to prevent the tungsten encroachment problem, it is still necessary, or at least desirable, to deposit the barrier metal on the sidewalls of the contact opening as well. This is because tungsten does not readily deposit on an insulating layer such as SiO2. Since the contact opening sidewalls are part of the insulating layer, a continuous barrier metal layer on the sidewalls is helpful to ensure conformal deposition of tungsten required to form a complete plug within the contact opening.
Hence, regardless of which metal (aluminum or tungsten) is used as a connection plug, the need for a continuous barrier metal layer on the sidewalls, especially in large aspect ratio contact openings, now about 3.5:1 to as much as 5:1 for advanced integrated circuits, is still present. Because of this, the industry has expended a great deal of effort in achieving conformal deposition of barrier metal in the contact opening. To this end, integrated circuit processing industry has recently developed a CVD of TiN process which provides good sidewall coverage of the contact opening and most manufacturers are moving toward CVD of TiN.
However, a good consistent barrier film in the contact opening which affords good adhesion to all surfaces and prevents encroachment at the bottom of contact openings presents a new difficulty. It is possible, in some instances, due to mask misalignment and other process variations, that the pattern of the metallic interconnection layer over a metal tungsten plug in the contact opening fails to completely cover every portion of that opening. In those cases, during etching of the metallic interconnection layer, the barrier metal exposed to the etching chemical will also be etched, which results in void formation or even etching of the barrier metal along the sidewalls of the contact opening.
Selective etching of tungsten relative to aluminum or titanium is easy to achieve. Tungsten, for example, is selectively etched with fluorine ions over titanium, titanium nitride, and aluminum. In addition, titanium, titanium nitride, and aluminum are selectively etched with chlorine ions relative to tungsten. Because of this etching selectivity, the tungsten plug within the contact opening can be etched back very uniformly and completely using titanium nitride, a barrier metal, as an etch stop. Then during the formation of an aluminum interconnection layer, for example, the tungsten plug is used as an etch stop for the aluminum. This aluminum etch process is relatively long due to the need to remove residual aluminum and titanium nitride near the contact opening.
Unfortunately, the chlorine etch preferred for etching aluminum also etches barrier metals such as titanium or titanium nitride. As a result, the barrier metal between the sidewall and the tungsten plug will also be etched when aluminum is etched. Because the etching period and over etch period for the aluminum metal is relatively long, the barrier metal on the sidewall can erode partially even towards the bottom of the contact opening (see FIG. 1, following). This may destroy the underlying conductive region such as a transistor source or drain located underneath the tungsten plug.
One method of preventing this problem is to make the interconnection layer of aluminum sufficiently large over the plug so that it completely covers and encloses the tungsten plug. A minimum enclosure defines the extra surface area which must be added to an interconnection layer in order to compensate for mask misalignment and other process variations. The widened portion over the plug may be typically approximately twice the interconnection layer width for small geometry devices. This enlarged area is designed to be centered on the interconnection layer, but it may be offset to one side and made sufficiently large to compensate for mask misalignment and other process variations. For example, if an interconnection layer has a width of 1 micron, the width of the region overlying the contact opening might be 2 microns to ensure complete coverage and enclosure of the tungsten plug and titanium sidewall (see FIGS. 2 and 3, following). It is disadvantageous to have wider interconnection layers or enlarged regions over a contact, especially in today""s integrated circuit devices in which the circuit elements and interconnection layers are packed more tightly than ever before; in today""s 0.35 micron technology devices, for example, the interconnection layers may be only 0.4-0.5 micron wide and stacked three to five levels deep.
Thus, it would be desirable to provide a contact opening that is not subject to erosion during formation of an interconnection layer. It would also be desirable to do this in such a way as to preserve much of the barrier metal deposited on the sidewalls so that deposition of a metal plug within the opening is conformal.
According to principles of the present invention, the contact opening in the insulating layer includes a bowl shaped sidewall portion and a straight sidewall portion. A thin layer of a barrier metal, such as titanium or titanium nitride, is deposited in the opening in contact with a conductive layer under the insulating layer. The thin layer of barrier metal covers the sidewall, both the bowl shaped sidewall portion and the straight sidewall portion. In addition, the barrier metal covers the upper surfaces of conductor at the bottom of the contact opening. A conformal conductive material is then formed within the contact opening, overlaying the barrier metal and filling the contact opening approximately to the top so that the top of the conformal conductive material is approximately coplanar with the upper surface of the insulating layer. The contact opening is larger in diameter at the top portion than it is at the bottom portion. The conformal conductive material thus has a larger diameter at the top of the contact opening and extends to cover, in a vertical alignment, the thin barrier layer positioned in the straight wall portion of the contact opening.
The conformal conductive material acts as a etch stop for the etch process which etches the barrier metal. The barrier layer and the conformal conductive material are selected such that each can be etched by different materials and one is the etch stop for the other in a selected etch process. For example, if a titanium material is used as the barrier layer, then tungsten is an appropriate choice for the conformal conductive material because tungsten can act as an etch stop to permit selective etching of the barrier metal of titanium which is covered by the tungsten. Other materials may also be selected for the barrier layer and the conformal conductive material which may be etched by different etch chemistries.
A second conductive layer is positioned on top of the conformal conductive material to provide an electrical interconnection from the second conductive material to the conductive layer under the insulating layer. The conformal conductive material is an etch stop for a process which etches the second conductive material. Generally, the second conductive material is an aluminum interconnection layer which overlays the conductive tungsten plug to provide ohmic contact between various circuit elements on the integrated circuit. Tungsten is an etch stop for aluminum.
According to the method of the present invention, a conductive layer is formed as part of an integrated circuit. An insulator layer is formed overlying the conductive layer. A mask is formed overlying the conductive layer and an opening is provided in the mask. An isotropic etch is performed with the mask in place to isotropically etch into the insulating layer through the opening. This etch will create an enlarged region, larger than the masked opening within the insulating layer because it is an isotropic etch. After the isotropic etch is conducted for a brief period of time, the etch is terminated. Subsequently, an anisotropic etch is carried out using the same mask opening. The anisotropic etch is approximately the same dimension as the opening and forms a straight sidewall portion of the contact opening. The anisotropic etch continues until the insulating layer is completely removed within the mask opening and the lower conductive layer is exposed.
A thin layer of barrier material is formed overlying the sidewalls of the contact opening, both the bowl shaped sidewall portion and the straight sidewall portion. The layer of barrier material also overlies the conductive material. A conformal conductive material is then deposited, overlying the thin layer of barrier material to sufficient thickness to fill the contact opening. A second conductive material is then deposited overlying the insulating layer and also overlying the conformal conductive material. The second conductive material is anisotropically etched to form a desired interconnection pattern. During the anisotropic etching of the second conductive material, the conformal conductive material acts as an etch stop to prevent etching of the barrier metal positioned vertically below it.
The invention provides the advantage that the second conductive material can be etched to form an interconnection layer without the need to ensure that it completely overlies all portions of the contact opening. The conformal conductive material, being selected as an etch stop for both the second conductive material and the thin barrier metal. The etched interconnection can be made sufficiently thin over the contact opening that it provides good electrical contact without the necessity to completely enclose the contact opening. Increased density for forming interconnection lines is permitted.